1. Field of the Invention
This invention relates to a process of producing sintered hard metals, in particular, cemented carbides and an apparatus therefor and more particularly, it is concerned with a method of sintering cemented carbides whereby the dispersion of qualities of products can be decreased, which is unavoidable in the vacuum sintering method commonly used for sintering cemented carbides.
2. DESCRIPTION OF THE PRIOR ART
Cemented carbides are produced ordinarily by the powder metallurgical method wherein powdered starting materials are mixed, pressed, if necessary subjected to presintering and machining and then subjected to final sintering at 1300.degree. to 1500.degree. C. to give a pore-free product. Of various dispersed properties of the product occurring during this production process, the quantity of carbon (which will hereinafter be referred to as "carbon content") requires the most precise control, since the allowable range of carbon content is less than 3% within which limit cemented carbides are capable of acting well and if the carbon content is low, a brittle .eta.-phase is precipitated, while if the carbon content is high, free carbon is precipitated. Furthermore, it is well known that a slight difference in carbon content has a marked influence upon the property of cemented carbides even within the allowable range.
The commonly used sintering method is a vacuum sintering method by which, however, it is very difficult to prepare cemented carbides with a stable carbon content. The reason thereof is as follows: Various starting material powders for cemented carbides are fine and the mixed powder (which will hereinafter be referred to as "starting mixed powder") obtained therefrom through ball milling by wet process has such a very fine particle size that the specific surface area amounts to several m.sup.2 /g. Such a fine powder, upon exposure to the air, is easily ozidized with oxygen and moisture contained in the air. One of the inventors has made detailed studies on this phenomenon [Akio Hara: Study on Oxidation of Starting Material Powders for Cemented Carbides in Air at Normal Temperature in "Funtai oyobi Funmatsu Yakin (Journal of the Japan Society of Powder and Powder Metallurgy)", Vol. 17, No. 8, page 338 (1971)]. According to this study, the oxidation phenomenon of a powder is a kind of rusting phenomenon forming hydroxides in the air. In the heating step of vacuum sintering, this hydroxide is reduced with carbon and, consequently, the carbon content of a compact is decreased during sintering, depending on the quantity of the hydroxide. Since formation of hydroxides tends to be affected by the weather, the carbon content in the product fluctuates with the weather. In order to prevent this phenomenon, handling of starting mixed powders and production facilities have hitherto been considered seriously, but dissipation of the quality of the product cannot be sufficiently suppressed by vacuum sintering.
A method for preventing the decrease of carbon content by reducing the hydroxides with hydrogen is effective in some aspects, but causes certain problems as follows:
The first cause is based on the following reducing reaction (1), EQU H.sub.2 +(O).fwdarw.H.sub.2 O (1)
in which (O) is oxygen contained in the hydroxides or oxides. The course of this reaction depends on the value of P.sub.H.sbsb.2.sub.O / P.sub.H.sbsb.2 and, therefore, it is necessary to raise the purity of hydrogen to be fed to a sintering furnace (i.e., to decrease H.sub.2 O) in order to accomplish the reaction sufficiently. Since H.sub.2 O is generated by the reaction, however, the hydrogen atmosphere has only a low reducing capacity even if high purity H.sub.2 is used unless H.sub.2 O is removed sufficiently rapidly. As described above, the starting mixed powders for cemented carbides are very fine, so a compact thereof is a porous body having very fine pores and hydroxides are considered to be formed on the walls of pores (inner surfaces). For the purpose of effecting the reducing reaction in the pores sufficiently, it is necessary to remove H.sub.2 O rapidly and to introduce H.sub.2 for the above described reasons. Using the ordinary hydrogen furnace, therefore, movement of H.sub.2 is not sufficiently carried out and reduction with H.sub.2 is not satisfactory.
The second cause is based on the following reaction (2) because a compact and presintered body are carbonaceous and structural parts of carbon are used in a furnace: EQU C+2H.sub.2 .revreaction.CH.sub.4 ( 2)
This reaction proceeds to the right at 600.degree. C. or lower under a total pressure of 1 atm, which corresponds to a decarburization reaction for a compact to be sintered, and proceeds to the left at 600.degree. C. or higher, which corresponds to a carburization reaction. It is difficult to keep uniform the temperature at all sites of the furnace and the flow of hydrogen gas results often in nonuniform conditions, so that both reactions of decarburization and carburization take place and the dissipation of quality is remarkable, depending on the sites in the furnace. Control or reduction of this problem is difficult.
The third cause is based on the following decarburization reaction (3) at 900.degree. C. or higher: EQU H.sub.2 O+C.fwdarw.CO+H.sub.2 ( 3)
In this case H.sub.2 O is also a problem, which is not introduced as an impurity in hydrogen, but is formed by the reducing reaction or separated from absorbed H.sub.2 O in the furnace.
The above described three reactions are in complicated relation with each other and control of these reactions, therefore, is more difficult than in the case of a vacuum furnace. In particular, the reactions (2) and (3) at high temperatures are unfavorably affected by the presence of hydrogen gas itself. Of late, therefore, the hydrogen sintering method has scarcely been employed.